Featuring among the essential qualities which may be expected of transmission equipment are of course its resistance to various disturbances and its capacity to transmit data error-free which are of major importance, and also its behaviour in the event of a breakdown.
It is customary to double up all or part of the equipment in order to minimize interruptions to service. In particular, it is conventional to cater for a failure of a supply source by using a second identical supply source connected in redundant mode with the first, that is to say connected in parallel with respect to the load which is connected to the output of the two sources. In practice, electrical energy converters are generally used as supply sources.
FIG.1 represents the structure of an item of conventional transmission equipment, which item comprises two energy converters 10 and 20 intended to supply three modules 30, 40 and 50 of the equipment. The two converters constitute the supply device of the equipment. The modules 30 and 50 are supplied by the converters 10 and 20 respectively while the module 40 is supplied by both converters at once. The converter 10 (respectively 20) is supplied between a positive-voltage terminal 11 and a negative-voltage terminal 12 (21 and 22 respectively). The converters are connected in parallel with the module 40, their coupling being effected through blocking diodes 13 (23) from whose cathode a DC supply voltage V0 stems. For this purpose, each converter comprises a common output terminal 14 (24) linked to the input of the module 40 and on which it delivers the supply voltage V0. Furthermore, each converter comprises a voltage regulator 15 (25) for example of the pulse width modulation (PWM) type. A slaving loop 16 (26) maintains the voltage on the common output 14 (24) at the value V0. Each converter also comprises a local output terminal 17 (27) for providing a DC supply voltage V1 (V2) to the module 30 (50) which is not backed-up. In the example of FIG. 1, a voltage V1 or V2 equal to V0 is sought. Accordingly, each converter comprises a diode 18 (28) connected between the output of the regulator and the local output terminal in order to restore on the latter the same voltage as on the common output.
Such a structure, although commonly used, nevertheless poses some problems. Thus, the two converters supply the module 40 simultaneously with a DC voltage V0 which is kept constant by the two slaving loops. The two slaving loops having a node in common, they therefore influence one another and may bring about a misbalance of operation between the two converters. Thus, if for example the converter 10 delivers a rising voltage V0, the converter 20 will detect this rise in the voltage V0 and the slaving loop 26 will react so as to lower the output voltage of the regulator 25. At some stage the blocking diode 23 turns off and, the slaving loop 26 being unable to operate, the output of the regulator falls to zero. If the converter 10 then suffers a failure, the converter 20 will start operating normally again, however there will be a transient period during which the module 40 will not be supplied. This period corresponding to an interruption of service of the equipment is not acceptable. Another drawback is that, in the above configuration, the converter 20 is not able to provide the voltage V2 expected on the local output 27 when its slaving loop cannot operate. Finally, such an arrangement ignores the fact that the voltage drop in the diodes of the device varies greatly according to temperature and may therefore introduce disparities between the voltages V0, V1 and V2 which are assumed to be equal in the example of FIG. 1.
To remedy these drawbacks, various solutions have been described in the state of the art. A known converter is described in FIG. 2. It differs from the converter 10 of FIG. 1 through its mechanism 16 for slaving the voltage of the common output. The slaving mechanism described in this document takes account of the state (off or on) of the blocking diode in order to control the regulator. To this end, the slaving mechanism 16 taps the voltage either side of the blocking diode 13 by means of two resistor bridges 31, 32 and 33, 34. The midpoint 35 of the resistor bridge 31, 32 is linked to the midpoint 36 of the resistor bridge 33, 34 via a diode 37. The cathode of this diode is connected to the node 35 while the anode is connected to the node 36. The node 36 is moreover connected to the inverting input of an operational amplifier 38 which additionally receives a reference voltage Vref on its non-inverting input. Finally, the output of the amplifier 38 is linked to a control input of the regulator 15.
This converter operates as follows: as long as the blocking diode 13 is on, a fraction of the common supply voltage V0 is compared with the reference voltage Vref in order to control the regulator 15. During this phase, the diode 37 is off. On the other hand, as soon as the blocking diode 13 switches off, the diode 37 comes on and the voltage on the node 36 will fall owing to the parallel mounting of the resistors 32 and 34. The operational amplifier 38 therefore receives a falling voltage on its inverting input and its output will therefore act on the regulator in such a way that it increases its output voltage and thus switches the blocking diode 13 back on.
Such a converter only partially solves the problems mentioned earlier. Thus, the voltage V1 or V2, delivered on the local output of the converter 10 or 20, of smaller rating is not sufficient when the latter's blocking diode is off or at its turn-off limit. Thus, if the blocking diode switches off, it is because the regulator is not delivering enough to yield the voltage V0 on its common output. Also, since the diodes 13 (23) and 18 (28) are identical, the converter is then unable to deliver enough voltage V1 or V2 on its local output. Moreover this converter does not solve the problem of the spread in the values of the voltage drop in the diodes as a function of temperature.